1. Field of the Invention
This invention relates to a slide fastener and has particular reference to a fastener stringer having a continuous plastic filament spirally formed and woven into a longitudinal edge of a stringer tape simultaneously as the latter is woven.
2. Prior Art
There have been proposed a number of slide fasteners or zippers of the type described, which comprise a woven stringer tape and a plastic filament woven into a longitudinal edge of the tape, the filament being usually formed into a helical coil structure composed of a succession of loops or elongated convolutions each having a coupling head, an upper and a lower leg and a connecting portion.
When bent into such helical coil formation, the filament which is thicker and harder than the weave yarn tends to yield back, thereby causing the fastener stringer to stretch longitudinally and causing the filament loops to tilt away from an upright posture with respect to the plane of the stringer tape. This displacement of the fastener and filament loops would result in irregular pitch of the filament loops, which in turn hinders smooth closing and opening operation of the slide fastener.
A means is disclosed in Japanese Patent Publication No. 46-7018 for eliminating the tendency of the individual coil loops to tilt away from their desirably upright posture, the means comprising a dense array of warp threads along a tape edge into which the filament is woven. This prior art, however, still has a drawback in that the fastener stringers are susceptible to longitudinal stretch and hence unstable in the loop-to-loop pitch of the filament.
Disclosed in Japanese Patent Laid-Open Publication No. 56-60504 is a woven fastener stringer having binding warp threads extending over every two coil loops and underlying a foundation weft in double pick between adjacent loops thereby forming a pad-like structure between the upper and lower legs of the loops so as to improve resistance to tortion of the filament loops and to stabilize the inter-loop pitch. This stringer structure having binding warp threads bundled up by weft threads would become extremely thick and heavy and hence less flexible.